1. Field of the Invention
The present invention relates to an optical parametric amplifier which amplifies a signal light parametrically using nonlinear optical effects. In particular, it relates to a technique designed for high gain and broadband parametric amplification.
2. Description of the Related Art
Currently, optical wavelength division multiplexing (WDM) transmission systems, in which a plurality of signal lights with different wavelengths is multiplexed for transmission by one optical fiber, are coming into practical use. On the other hand, in recent years, attention has been given to optical time division multiplexing (OTDM) transmission systems, in which the bit rate per wavelength is increased and signal lights are multiplexed by time division, due to problems such as the increase in power consumption caused by increasing numbers of wavelengths.
In the OTDM transmission system, since the bit rate is high, an optical switch that controls the on-off switching of a high speed signal light that cannot be processed electrically as it is in the optical state, and an optical sampling system for measuring optical pulses, have become essential basic element technologies.
In recent years, the nonlinearity of optical fibers has been improved compared with the past, and applications are proposed of an ultra high-speed optical switch and an optical sampling technique, in which nonlinear optical effects of optical fibers are used positively. The above-described nonlinear optical effects mean phenomena in which, for example when light with a relatively high power is transmitted through glass, the physical properties of the glass change according to the optical power, and the optical response loses linearity.
Parametric amplification, which is generated in a highly nonlinear fiber having particularly high nonlinearity, is a physical optical phenomenon generated at extremely high speed. Therefore, the speed of response is far higher when compared with a phenomenon in which the refractive index of a medium is changed by thermo- and electro-optical effects, so that it is possible to respond to the high speed optical pulses used in OTDM with no delay. In this manner, the optical parametric amplification technique is expected to be an optical amplification principle with high speed response characteristics, which is essential to constructing future ultra high-speed networks.
For a conventional optical amplifier in which parametric amplification as described above is used, a broadband nonlinear polarized amplifier is proposed for example in Japanese Patent Application National Publication (Laid-Open) No. 2002-50861, in which a combination of cascading Raman amplification, and either one of parametric amplification in an optical fiber or four wave mixing, is used. In the conventional technique, one intermediate order for cascading Raman amplification is arranged such that it is close to the zero-dispersion wavelength of an amplification fiber, and broadband optical amplification is achieved by utilizing the fact that, in the case where the wavelength is longer than the zero-dispersion wavelength, it is phase-matched by parametric amplification, and in the case where it is shorter than the zero-dispersion wavelength, it is phase-matched by four wave mixing.
Furthermore, for another conventional technique in which nonlinear optical effects of optical fibers are utilized, a system is also proposed for example in Japanese Unexamined Patent Publication No. 2000-180807, that performs wavelength conversion of a signal light by four wave mixing by injecting, into a plurality of dispersion-shifted optical fibers each having different zero-dispersion wavelengths and connected in series, a signal light, and a pump light with the same wavelength as one of the different zero-dispersion wavelengths.
Incidentally, as is apparent from the fact that the above-described conventional optical amplifiers realize broadband by combining cascading Raman amplification and parametric amplification or four wave mixing, regarding an optical parametric amplifier that amplifies a signal light parametrically, there is a problem in that it is difficult to realize the desired bandwidth and gain in the case where a signal light is amplified using parametric amplification on its own.
Here is a description of the basic principle of operation of an optical parametric amplifier.
In a typical optical parametric amplifier, for example as shown at the top of FIG. 7, a signal light pulse S with a wavelength λs, injected from a signal light source 101, and a pump light pulse P with a wavelength λp, injected from a pump light source 102, are applied to a nonlinear fiber 106 via polarization controllers 103 and 104, and a coupler 105, and light transmitted through the nonlinear fiber 106 is passed through an optical bandpass filter 107, and the resultant parametrically amplified signal light S, is output externally. The pump light wavelength λp is set such that it coincides with the zero-dispersion wavelength λ0 of the nonlinear fiber 106 (λp=λ0) as shown at the bottom of FIG. 7. In the figure, Δλ denotes the distance (separation amount) between the signal light wavelength λs and the pump light wavelength λp. In an optical parametric amplifier with such a construction, the following items can be given as factors that govern the generation of parametric gain.
(i) Factors originating from four wave mixing
(ii) Factors originating from the walk-off between a signal light S and a pump light P
Therefore, in order to realize parametric gain broadband, high parametric gain, it is necessary to consider the factors (i) and (ii) when producing a design.
To be specific, regarding the factors in (i), the four wave mixing generation efficiency and phase matching conditions are important. Four wave mixing efficiency ηc is typically represented by the following equation (1).ηc=[[1−exp(−αz)]/α]2 exp(−αz)[γPp]2  (1)
Here, α denotes the absorption coefficient of a nonlinear fiber, γ denotes the nonlinear coefficient, and Pp denotes the optical power of the pump light pulse P. From the relationship in the above-described equation (1), in order to ensure the required four wave mixing efficiency, it is necessary to suppress the influence of the absorption coefficient α, and increase the pump optical power Pp.
Furthermore, the phase matching condition Δβ is satisfied when the following equation (2) is satisfied, which is represented using a signal light wavelength λs, a pump light wavelength λp, the zero-dispersion wavelength λ0 of the nonlinear fiber 106, and the dispersion slope dDc/dλ.
                                          Δ            ⁢                                                  ⁢            β                    =                                    -              2                        ⁢                                                  ⁢            γ            ⁢                                                  ⁢                          P              p                                      ⁢                                  ⁢                              Δ            ⁢                                                  ⁢            β                    =                                    -                                                2                  ⁢                                                                          ⁢                  π                  ⁢                                                                          ⁢                  c                  ⁢                                                                          ⁢                                      λ                    0                    3                                                                                        λ                    P                    3                                    ⁢                                      λ                    S                    2                                                                        ⁢                                          ⅆ                                  D                  C                                                            ⅆ                λ                                      ⁢                                          (                                                      λ                    P                                    -                                      λ                    S                                                  )                            2                        ⁢                          (                                                λ                  P                                -                                  λ                  0                                            )                                                          (        2        )            
As described previously, in a typical optical parametric amplifier, a design is produced in which the zero-dispersion wavelength λ0 of a nonlinear fiber and a pump light wavelength λp coincide. However, since the zero-dispersion wavelength λ0 of the nonlinear fiber actually used cannot avoid fluctuation in the longitudinal direction, it is difficult to realize the state of λ0=λp exactly. If the phase matching condition of equation (2) described above is satisfied in the state in which the pump light wavelength λp is shifted relative to the zero-dispersion wavelength λ0, the maximum parametric gain will be achieved at a specific wavelength. That is, the parametric gain in the ideal state of λ0=λp becomes constant relative to wavelength over a wide range, but a peak occurs in the bandwidth characteristic of the parametric gain actually obtained, due to the shift of the pump light wavelength λp relative to the zero-dispersion wavelength λ0. The occurrence of the peak limits the bandwidth of the optical parametric amplifier. Moreover, in order to amplify a desired signal light by a high gain, it is necessary to set the zero-dispersion wavelength λ0 and the pump light wavelength λp such that the peak wavelength of the parametric gain appears close to the signal light wavelength. However, this is not easy to realize due to the aforementioned fluctuations and the like of the zero-dispersion wavelength λ0.
When the dispersion slope is zero (dDc/dλ=0), the relationship of equation (2) is not satisfied, and hence phase mismatching always remains, causing a decrease in the gain.
Considering the relationship between equation (1) and equation (2), in order to realize high gain in an optical parametric amplifier, for example two wavelength excitation using pump lights of two wavelengths is effective. However, since an optical parametric amplifier using two wavelength excitation requires literally two pump light sources, there is a problem in that it has a cost disadvantage.
Regarding the factors in (ii) originating from walk-off, the walk-off (delay amount) Δτ between the signal light S and the pump light P is obtained from the following equation (3) using the length L of the nonlinear fiber.
                              Δ          ⁢                                          ⁢          τ                =                                            ⅆ                              D                C                                                    ⅆ              λ                                ⁢                                    (                                                λ                  P                                -                                  λ                  S                                            )                        2                    ⁢          L                                    (        3        )            
FIG. 8 illustrates the relationship between the parametric gain and the walk-off Δτ between the signal light S and the pump light P. This relationship shows that when the walk-off Δτ reaches a certain value, the parametric gain reaches a maximum. The decrease in gain in the region where the walk-off Δτ is comparatively low, before the parametric gain reaches its maximum, indicates phase mismatching. On the other hand, the decrease in gain after the parametric gain reaches its maximum is caused by the walk-off Δτ between the signal light pulse S and the pump light pulse P increasing. In this manner, in parametric amplification, the walk-off Δτ between the signal light pulse S and the pump light pulse P, and the phase matching conditions Δβ, are factors that have a great influence over the gain.